QKD involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using either single-photons or weak (e.g., 0.1 photon on average) optical signals (pulses) called “qubits” or “quantum signals” transmitted over a “quantum channel.” Unlike classical cryptography whose security depends on computational impracticality, the security of quantum cryptography is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits introduced errors that reveal her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett (“the Bennett Patent”), which patent is incorporated by reference herein, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992), which article is incorporated herein by reference. The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33.
Entanglement-based Quantum Communication (QC) and, in particular, QKD, is one of the most promising applications of Quantum Information Science (QIS). A number of technical challenges, however, must be overcome before QKD becomes commercially practical. One of these challenges includes developing high-photon-flux entanglement sources for telecommunication wavelengths (e.g., 1550 nm). Because of the low attenuation of 1550-nm photons in optical fibers, they are natural information carriers for long-distance communication links.
Because quantum communication is based on single photon exchange and detection, it is very important to minimize the number of “unintended” photons—that is to say, the photons that do not participate in the information exchange. Therefore, deploying QKD system in an existing optical fiber communication infrastructure populated by classical communication channels requires very aggressive spectral filtering to minimize the background noise and optimize the signal-to-noise ratio (SNR) at the receiver.
For optimal quantum communication system performance, the width and the shape of the receiver bandpass filter must match the transmitted signal. Narrowing the transmitter spectrum decreases the receiver spectral bandwidth. Since the received noise is proportional to receiver spectral bandwidth, using narrow-line transmitters results in higher SNR, which extends the quantum channel distance budget.
Spontaneous parametric processes naturally produce relatively broadband emission spectra, typically on the order of a few nm to few tens of nm. A broad signal spectrum can cause the signals to spread due to dispersion in fiber-optics-based QKD system. Hence, additional filtering is required. One way to perform such filtering is to use an external bandpass filter at the transmitter output. Unfortunately, this decreases the photon flux and overall link efficiency, which reduces the performance of the quantum communication system.